Using SASS to calculate lengths in em from lengths specified in px.

gistfile1.scss

// Ems relative to the typical 16px default body font-size.
$BASE-PX: 16;

// Default $em value, which can be overridden in narrower scopes.
$em: 1em / $BASE-PX;

// Writes a font-size declaration with EM units based on the pixel measurement
// $size-px, overriding the $em unit variable temporarily for any included
// @content block.
@mixin rel-font-size($size-px, $relative-to-px: $BASE-PX) {
  $_m: $em;
  $em: 1em / $size-px;
  font-size: 1em * $size-px / $relative-to-px;
  @content;
  $em: $_m;
}

.before-rel-font-size {
  font-size: 14 * $em;          /* 14 / 16 = 0.875em */
  line-height: 20 * $em;        /* 20 / 16 * 14 = 17.5 */
}


*, .md-body-1 {
  @include rel-font-size(14) {  /* 14 / 16 = 0.875em */
    letter-spacing: 0.010em;
    line-height: 20 * $em;      /* 20 / 14 * 14 = 19.99998 */
  }
}

.after-rel-font-size {
  font-size: 14 * $em;          /* 14 / 16 = 0.875em */
  line-height: 20 * $em;        /* 20 / 16 * 14 = 17.5 */
}